Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations, consisting of driven piles or drilled concrete piers. The deep foundations are designed to transfer the structure loads through the soft soils to more competent soil strata.
In recent years, aggregate piers have been increasingly used to support structures located in areas containing soft soils. The piers are designed to reinforce and strengthen the soft layer and minimize resulting settlements. The piers are constructed using a variety of methods including the drilling and tamping method described in U.S. Pat. Nos. 5,249,892 and 6,354,766 (“short aggregate piers”), the driven mandrel method described in U.S. Pat. No. 6,425,713 (“Lateral Displacement Pier), and the tamper head driven mandrel method known as the “Impact Pier” as disclosed in published U.S. patent application, Pub. No. U.S.2004/0115011, dated Jun. 17, 2004.
The Short Aggregate Pier method (U.S. Pat. Nos. 5,249,892 and 6,354,766), which includes drilling or excavating a cavity, is an effective foundation solution when installed in cohesive soils where the sidewall stability of the hole is easily maintained.
The Lateral Displacement Pier (U.S. Pat. No. 6,425,713) and Impact Pier (U.S. patent application, Pub. No. U.S.2004/0115011) methods were developed for aggregate pier installations in granular soils where the sidewall stability of cavities is not easily maintained. The Lateral Displacement Pier is built by driving a pipe into the ground, drilling out the soil inside the pipe, filling the pipe with aggregate, and using the pipe to compact the aggregate “in thin lifts”. A beveled edge is used at the bottom of the pipe for compaction. The Impact Pier covers an extension of the Lateral Displacement Pier. In this case, a smaller diameter (8 to 16 inch) tamper head is driven into the ground. The tamper head is attached to a pipe, which is filled with crushed stone once the tamper head is driven to the design depth. The tamper head is lifted allowing stone to fall into the cavity and then the tamper head is driven back down densifying each lift of aggregate. One advantage of the Impact Pier is the speed of pier construction.
To supply sufficient aggregate for the completion of the pier, the methods for both the Lateral Displacement Pier and the Impact Pier require either that a hopper, located at the top of the pipe or mandrel, be filled and lifted with the pipe or mandrel as part of pier installation activities, or that an aggregate delivery system be implemented to raise aggregate to the top of the pipe or mandrel during installation activities. Both the use of a raised hopper and the use of an aggregate delivery system add complexity and costs to the pier construction process.